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New study looks at ecological “tipping points” for coastal species to help manage for change

A new paper published by a team of scientists from the U.S. Geological Survey and U.S. Fish and Wildlife Service in Ocean & Coastal Management synthesizes existing information on ecological thresholds related to environmental changes -- including sea-level rise and coastal storms -- for 45 species of coastal fish, wildlife, and plants selected because of their ecological, economic and cultural importance.

Diane Nunley/USFWS

Coastal systems like salt marshes, mangrove forests, and barrier beaches are home to a diversity of wildlife species, such as whooping crane, Eastern oyster and American black duck.

These species are important parts of functional ecosystems that contribute a host of economic, recreational and cultural benefits to people and communities. For example, the Eastern oyster is a cornerstone of a lucrative shellfish industry, filters water as it feeds, and helps stabilize the shoreline by creating reefs. Healthy coastal marshes are the nursery areas for fish species that support critical recreational and commercial coastal fisheries.

Some species also play an instructive role as indicators of environmental changes that can threaten coastal systems that people and wildlife depend upon. These indicator species may experience "ecological thresholds" - the tipping points at which changing environmental conditions will lead to a disruption in a species life cycle or habitat. By looking at ecological thresholds for key species, we can better prepare for and respond to threats that can affect their homes - and ours.

A team of scientists from the U.S. Geological Survey and U.S. Fish and Wildlife Service collaborated to synthesize existing information on ecological thresholds related to environmental changes, including sea-level rise and coastal storms, for 45 species of coastal fish, wildlife, and plants selected because of their ecological, economic and cultural importance.

The synthesis responds to a need identified by stakeholders from six LCCs, three CSCs, state wildlife agencies and other partners from across the U.S. Atlantic and Gulf Coast regions for information on thresholds that can help managers act strategically despite uncertain future conditions. This information is timely for helping coastal states implement updated State Wildlife Action Plans (SWAPs), which identify species of greatest conservation need in each state and outline strategies for protecting them in the face of challenges. All of the SWAPs in coastal Northeast states identify environmental impacts to species in the coastal zone as one of the greatest and most actionable shared threats in the region.

“In the latest generation of SWAPs, we focused on setting measurable objectives for species, so having quantitative information on thresholds helps us know if we are moving in the right direction,” said Amanda Shearin, Maine’s Wildlife Action Plan coordinator. “The fact that this study was based on regional collaboration makes it that much more valuable. Coastal change is a big picture issue, and being able to look at impacts across the region is so useful, especially when it gives us guidance that we can apply locally.”

All of the species in the paper had either been identified previously as vulnerable to sea-level rise or storms, as representative species for others with similar ecological needs and sensitivities, or as Species of Greatest Conservation Need in SWAPs. Some, like saltmarsh sparrow, fulfill all three criteria.

The synthesis revealed that there are disparities in the quality and quantity of information that is available to help managers act strategically to protect these species and others with similar habitat needs from changing environmental conditions.

Canary in a coastal marsh: Mottled duck depend on saltmarsh habitat but are sensitive to changes in salinity that may result from sea-level rise. Photo: Dick Daniels

While there is information on ecological thresholds for more than half of the focal species (see table below), the information ranges from estimates of the percentage habitat loss or gain under different sea-level-rise scenarios, to numeric metrics that indicate how a species will respond to a specific environmental change.

For example, salinity levels greater than 9 parts per thousand can negatively affect the growth and survival of the mottled duck, a species that depends upon marshes along the Gulf Coast for nesting habitat. Sea-level rise can increase the salinity of coastal marsh habitats through greater tidal intrusion, and more frequent and severe coastal storms can also increase inundation and salinity. Therefore, protecting or creating brackish marsh habitat with salinities less than 9 parts per thousand is important for the survival of the mottled duck and other species with similar habitat requirements.

Although the numeric metrics offer a precise way to determine when a threshold has been crossed, percentages of habitat loss or gain in response to different levels of sea-level rise, known as “scenarios,” still represent valuable opportunities to inform where and how managers can take meaningful action - even in the face of uncertainty (see figure below).

While robust decision making is best supported by two or more threshold metrics across life stages or geographical regions within a species’ range, effective strategies can still be crafted when only a single threshold metric is available. The authors discuss a variety of coastal adaptation strategies in the paper to illustrate how threshold data can be applied to help increase the persistence and resilience of vulnerable coastal species.

“The more we can support our decision making with quantitative metrics, the more confidence we can have in taking action, even under uncertainty about exactly how much sea-level rise is expected over different timelines,” said lead author Emily Powell, the Coastal Research Associate for the North Atlantic LCC and Northeast CSC fellow. “This synthesis is a starting point for figuring out what additional research could be done to increase the availability and application of quantitative data to reduce risks to vulnerable coastal species and their habitats.”

Table 1: Data available by taxonomic group: Summary of the total number of species per major taxonomic group with and without quantitative threshold data related to sea-level rise and coastal storms.

Figure 1: Species' relative vulnerability to sea-level rise: A snapshot of impacts from sea-level rise (SLR) within marsh, beach/barrier island, mangrove, and shellfish-bed habitats based on a literature review of species thresholds. Locations of the species/guilds along the vertical axis represent approximate levels of SLR at which species are expected to be impacted according to the literature. Species are listed as being relatively more (bottom) to less (top) vulnerable within each group. Two species may experience habitat gains associated with SLR, denoted by a “+”, and may be most resilient to SLR.